1. Field of the Invention
The present invention relates to an optical transmitter in an optical communication system.
2. Description of the Related Art
Recently, with an increasing demand for a 40 Gbit/s next-generation optical transmission system, a transmission distance and frequency use efficiency equivalent to a 10 Gbit/s system are demanded. As realizing means, various systems (duobinary, CS-RZ (carrier suppression-return to zero), DPSK (differential phase shift keying), DQPSK (differential quadrature phase-shift keying), etc.) that excel the NRZ modulation system (non return to zero) conventionally applied in the system of 10 Gb/s or less in frequency use efficiency, resistance to optical-signal-to-noise ratio, and resistance to nonlinearity have been studied actively.
Especially, the DQPSK modulation system simultaneously transmits two phase-modulated digital signals using optical light of one frequency. In this system, the pulse iteration frequency can be half (for example, 20 GHz) the data transmission rate (for example, 40 Gbit/s). Therefore, the signal spectrum width can be half the value in the conventional NRZ modulation system and the like, and excels in frequency use efficiency, resistance to wavelength dispersion, device transmission characteristic, etc. Accordingly, in the field of the optical transmission system, especially in the high-speed optical transmission system exceeding the data transmission rate of 40 Gbit/s, this modulation system is widely studied for implementation.
In the optical transmitter for realizing the high-speed optical transmission system, a Mach-Zehnder type LN modulator is used (non-patent document 1). The transmission unit in the optical transmission system using these units requires a stabilizing technology for the parts of the transmission unit for stabilizing an optical transmission signal.
For example, a stabilizing technology of for the parts of the transmission unit can be an automatic bias control (ABC) circuit for preventing the degradation of a transmission signal by the drift of an LN modulator in the NRZ modulation system adopted in the system operating on land or on the seafloor as a practical system (patent document 1 and the like). In the NRZ and RZ modulation systems, the modulation unit for performing modulation with an electrical signal having the amplitude of Vπ using peak to valley or valley to peak of the drive-voltage-to-optical-intensity characteristic of the LN modulator is included. In the CS-RZ modulation system, the optical duobinary modulation system, the DPSK modulation system, and the DQPSK modulation system, the modulation unit for performing modulation with an electrical signal having the amplitude of 2×Vπ (Vπ indicates the voltage varying the phase of the modulator by π) using the peak, valley, and peak of the drive-voltage-to-optical-intensity characteristic is included.
FIG. 1 shows the configuration of realizing the LN modulator bias control by the Vπ electrical signal drive. FIG. 2 shows the configuration of realizing the LN modulator bias control by the Vπ electrical signal drive.
In the bias control device of (1) shown in FIG. 1, the light emitted from a laser diode 10 is input to a Mach-Zehnder (MZ) type modulator 11, intensity-modulated, and input to an optical coupler 12. In the optical coupler 12, a part of the light is branched, and received by a photodiode 13. The photodiode 13 converts an optical signal to an electrical signal, passes through an electric amplifier 14, and input to a synchronization detection unit 15. The synchronization detection unit 15 receives an oscillated wave from the oscillator of the low frequency f0, and the synchronization of a signal from the electric amplifier 14 is detected. A synchronization detection result is input from the synchronization detection unit 15 to a bias supply circuit 16. The bias supply circuit 16 performs bias control on the basis of the synchronization detection result. The low frequency f0 oscillated from an oscillator 18 is input to a modulator driver 17 with an electrical signal input Q, a drive signal obtained by superposing the low frequency f0 on the input signal Q is generated, and applied as the drive signal to the Mach-Zehnder type modulator 11.
(2) shown in FIG. 1 shows the input electrical signal and the characteristic of the MZ type modulator. The status of A is the optimum status of the bias, and (b) and (c) indicates the characteristic of the MZ type modulator when the bias voltage is shifted. In the status of the bias of (a) through (c), when a signal obtained by amplitude-modulating the low frequency signal having the frequency of f0 is input to the input electrical signal, the modulated optical signal indicated by (3) shown in FIG. 1 is obtained. In the optimum status of A, the low frequency component of the frequency f0 does not occur in the modulated optical signal. On the other hand, when the bias voltage of (b) and (c) is shifted, the low frequency component of the f0 component occurs in the modulated optical signal. The statuses (b) and (c) can be discriminated from each other by the 180° difference in phase of the low frequency component.
In (1) shown in FIG. 2, the same component as in (1) shown in FIG. 1 is assigned the same reference numeral, and the explanation is omitted here. In (1) shown in FIG. 2, the dual MZ type optical modulator is used, and the electrical signal inputs Q, Q− are applied to each of the two branched waveguides through the drivers 17-1 and 17-2. In this case, the bias voltage is changed at the low frequency of the frequency f0, and the fluctuation of the amplitude of the obtained optical signal is detected. As shown in (2) and (3) shown in FIG. 2, in A where the optimum bias voltage is obtained, no f0 component occurs in the output optical signal. However, when the bias voltage is shifted, the f0 component occurs in the output optical signal. (b) and (c) can be discriminated from each other by the difference in phase of the f0 component. Thus, FIG. 2 is basically the same as FIG. 1 except that the amplitude of the drive signal for driving the modulator is 2×Vπ and that the low frequency signal is not superposed, but the bias voltage itself is modulated.
The control signal of the frequency of f0 is superposed on the amplitude or the bias of the drive signal. Then, when the bias is shifted from the optimum value, the f0 component occurs in the detection signal. Since the phase of the f0 component is inverted depending on the shift direction from the optimum bias point, the direction of the bias shift can be detected.
Furthermore, as another parameter that determines the quality of the optical transmitter, there is the drive amplitude of an LN modulator. The method of controlling the drive amplitude to Vπ or 2×Vπ is not specifically presented. In the present system, control is not specifically performed, but the unit variance of a device is regarded as a margin or the waveform of a high-speed optical signal is observed at the initial adjustment, thereby performing the adjustment. However, a measurement unit for monitoring the waveform of the high-speed optical signal is required for the latter, and a resultant adjustment system is costly.
The patent document 2 features an optical frequency change amount detection device for detecting the amount of change in optical frequency of the output light, and a device for adjusting the drive condition of the optical modulator such that the amount of fluctuation of an optical frequency can be the optimum.
The patent document 3 features the adjustment between the drive amplitude and the phase by a phase comparison circuit and a phase comparison circuit to minimize the optical wavelength chirp to be provided for the transmission optical signal.
[Non-patent Document 1] Magazine FUJITSU, 54, 4, p. 314-322 (07, 2003)
[Patent Document 1] Japanese Patent Application Publication No. H3-251815
[Patent Document 2] Japanese Patent Application Publication No. H11-30517
[Patent Document 3] Japanese Patent Application Publication No. 2002-23124
FIGS. 3A and 3B show an example of the configuration of performing modulation with electrical signals of the drive amplitude of Vπ and 2×Vπ.
FIG. 3A shows the case in which the drive amplitude of Vπ is used, and FIG. 3B shows the case in which the drive amplitude of 2×Vπ is used. The configuration of the drive portion of Vπ and 2×Vπ included in each modulation system is unchanged. In each modulation system, when a shift from the optimum drive amplitude (level of drive amplitude) occurs by the variance of the electrical signal drive system, the degradation with time, and a temperature change, the quality of the transmission of an optical signal is degraded. Therefore, a configuration for monitoring the shift from the optimum point of the electrical signal drive amplitude, and controlling the electric drive signal amplitude is required.
FIGS. 4A and 4B are explanatory views using an example of calculating the degradation of the quality of a signal with respect to the drive amplitude. FIG. 4A shows the DQPSK optical transmitter used in the calculation. The signals of the data 1 and 2 output from the DQPSK signal source are amplified by the drivers 1 and 2, and drive the modulator. FIG. 4B shows the calculation result of the degradation of the quality of a signal with respect to the drive amplitude. The amount of degradation (Q penalty) of the signal quality is the lowest in the vicinity of Vπ, and the quality of a signal is degraded by a shift of the drive amplitude from the vicinity of Vπ of the modulator. This tendency appears more conspicuously when the coding method is a multivalue method than it is a binary method. Since the binary coding method has conventionally been used, the signal degradation in an allowable range can be rejected in the multivalue coding method.
As described above, to suppress the degradation of the quality of a signal, it is necessary to adjust or control the drive amplitude of each driver to be set around the vicinity of Vπ of each modulator. Especially, when the optical transmitter has a plurality of drivers for driving the modulators that are a plurality of Mach-Zehnder type modulators using a differential phase modulator when multivalue modulation such as the RZ-DPSK, the RZ-DQPSK, etc. is used, the adjustment is very hard. Therefore, a simple adjustment/control method is required.
However, there has not been any device for satisfying the above-mentioned requirements. In addition, as described above in the Description of the Prior Art, there is a bias control method, but the frequency f0 component of the control signal is constantly 0 (zero) regardless of the amount of drive signal amplitude. Accordingly, it cannot be applied to amplitude control of a drive signal.